The present invention concerns improvements to an electromechanical filter which is suitable for removing a fixed frequency carrier wave from amplitude modulated signals of similar frequencies with which the said wave is mixed.
Such a filter may be used, as a non-limiting example, in a carrier current multiplex telephone system. Let us assume that the frequency of the wave which is to be separated is close to 128 kHz, the bandwidth of the filter is 50 to 100 Hz. This frequency will hereinafter be called the "useful frequency".
More specifically, the present invention relates to a filter which is not only able to separate the useful frequency, but which provides a considerable attenuation at any frequency in the neighbourhood of the frequency band centred on the useful frequency and also at frequencies more remote from the said useful frequency. In other words, the invention relates to a filter having a single pass band.
Electromechanical filters usually consist of the following parts:
A PLURALITY OF RESONATORS DISPOSED NEXT TO ONE ANOTHER EACH FORMED OF A CYLINDRICAL METAL BAR VIBRATING IN COMPRESSION-EXTENSION ALONG ITS LENGTH, WHICH IS SO SELECTED AS TO BE TUNED AT THE USEFUL FREQUENCY AS A FUNDAMENTAL RESONANCE MODE, THE LONGITUDINAL AXES OF THE BARS BEING PARALLEL AND SUBSTANTIALLY SITUATED IN A COMMON PLANE;
AN ELECTROMECHANICAL INPUT TRANSDUCER WHICH COUPLES AN EXTERNAL ELECTRIC CIRCUIT TO ONE OF THE RESONATORS OF THE SAID PLURALITY WHICH WILL HEREINAFTER BE CALLED FOR THE SAKE OF SIMPLICITY THE "FIRST RESONATOR";
A MECHANICOELECTRICAL OUTPUT TRANSDUCER WHICH COUPLES ANOTHER EXTERNAL CIRCUIT TO ANOTHER ONE OF THE SAID RESONATORS, WHICH WILL HEREINAFTER BE CALLED THE "LAST RESONATOR";
AND SUBSTANTIALLY RECTILINEAR COUPLING RODS WHICH ARE PERPENDICULAR TO THE AXES OF THE RESONATORS, TO WHICH THEY ARE CONNECTED AT POINTS SUBSTANTIALLY MIDWAY ALONG THE LENGTH THEREOF, THE SAID RODS VIBRATING IN COMPRESSION-EXTENSION ALONG THEIR OWN LENGTH.
It is known to use as the transducers plates of piezoelectric ceramic material, for example of the lead-zirconiumtitanium (PZT) type, which, when a variable electric field is applied perpendicularly to their thickness, vibrate in a direction parallel to the planes of their principal faces and which conversely, in the case where they vibrate under the action of a mechanical force, develop between their opposite faces an electric field perpendicular to the latter.
Experience has shown that, although it seems at first sight that a unit such as that just defined constitutes a filter having a single narrow pass band centred on the single longitudinal resonance frequency common to the various resonators, such a unit will transmit other frequency bands between the input and output transducers which will hereinafter be called "parasitic bands" and which correspond to modes of vibration of the resonators other than the fundamental mode. Such parasitic bands are related to the mode of coupling of the transducers to the bars and are more particularly associated with flexural vibrations of the resonators, which cannot be avoided with current practice mounting arrangement between resonators and transducers.